1. Field of the Invention
The present invention relates to an apparatus for extracorporeal lung assist (ECLA) utilizing a membrane artificial lung and which is able to automatically control blood suction and feed by expanding and contracting a blood reservoir through liquid pressure. Unlike a pump oxygenerator which artificially performs lung functions for a short period of time, the apparatus in accordance with the invention provides respiratory aids safely for long (i.e., over a period of several days or even several weeks) and therefore it is useful for treatment of acute respiratory failure cases.
2. Description of Prior Art
Membrane artificial lungs employed in treating acute respiratory failure cases may be classified into three types, namely, coil type, plate type, and hollow fiber type, according to their structural characteristics. Such artificial lung is usually employed either in a double pump system or in a single pump system. The single pump system is advantageous over the double pump system in that the entire circuit involved is smaller in length, and in that less priming volume is involved, and further in that the system is easier to operate and less liable to danger due to malfunction. Yet, the single pump system leavs much to be desired. That is, a single-pump system apparatus which involves a further reduced priming volume is strongly demanded, more particularly for use with newborn and infant cases.
Living-body side conditions during extracorporeal blood circulation are not constant and blood outflow conditions are likely to change momentarily; however, tubings used for circuit forming purposes, unlike blood vessels, have no content-regulating capability. In apparatuses of the type, therefore, a system is generally employed such that a blood take-off circuit in which a blood feed pump is disposed through a blood reservoir which serves as a buffer against any volumetric change between the blood take-off circuit and the pump is connected to the in-flow side of an artificial lung, with a blood feed circuit connected to the out-flow side of the artificial lung, so that extracorporeal blood circulation is carried out by means of the pump.
Where such apparatus is employed, however, collection of blood into the blood reservoir whose function is to reserve any surplus of the blood in extracorporeal circulation which increases in volume with the progress of blood take-off is made through blood taking as effected with a head provided between the patient and the blood reservoir, and since the head is usually of more than 1 m. It is necessary that the patient must be placed at an elevated position for blood take-off purposes. Naturally, it follows that a longer circuit is required for blood collection; and it is inevitable that a larger priming volume is involved. Further, roller-type or finger type pumps which are usually employed as blood feed pumps have a drawback that squeezing or crushing of the tube by the rollers or fingers has considerable effects upon hematic breakdown and more particularly upon thrombocytopenia and hemolyzation. To replace such roller- or finger-pump, therefore, various types of pumps have been developed which are of such system that a flexible bag, housed in a rigid housing, is expanded and contracted by air pressure. However, all of these are merely of a pulsation type, instead of the conventional roller-or finger-type, and are not significantly contributive toward priming volume reduction.
Another difficulty with the conventional apparatus is that the circuit therein is so complicated that many connectors are used. During any prolonged extracorporeal blood circulation, however, blood is liable to coagulation; therefore, the number of connectors used must be as small as practicable.